In radio communication of transmitting and receiving digital data, the limit of a radio communication range is determined by transmission power of radio wave, the performance of antennas used in transmitting and receiving data, and a transfer rate. An increase in transmission power is directly associated with power consumption of a transmitter, and there is a limit on increasing the transmission power. The performance of antennas can be improved by using Yagi-Uda antennas or the like. However, the higher the performance of antennas, the larger the size and the more the structure becomes complex. Thus, the performance of usable antennas is limited.
Moreover, the transmission power is regulated by the Wireless Radio Act. Further, the antenna performance and the transmission power are regulated by the Wireless Radio Act depending on the frequency band of radio waves. As a result, there is a limit on the practical transmission power and antenna performance.
Direct sequence spread spectrum (DSSS) is known as a technique for realizing long-range radio communication under such restricts (for example, see NPL 1). DSSS is a technique of realizing high receiver sensitivity while eliminating the influence of noise by integrating received signals while multiplying the received signals by spreading codes. The sensitivity increases linearly by increasing the integration period (that is, lowering the transfer rate), and a global positioning system (GPS) that employs the DSSS scheme can receive signals stably in an electric field intensity of −150 dBm, for example.
A GPS transmits radio waves continuously with a stable phase. Thus, by employing a low-frequency phase locked loop (PLL) or delayed locked loop (DLL), stable phase synchronization is established in a low signal-to-noise ratio (SNR) state. If the phase is correct, weak signals can be detected by integrating and detecting the signals. When a dedicated radio frequency band as in GPS is allocated, it is possible to transmit signals continuously for a long period of time and to receive weak signals stably.
For example, a system that transmits information of sensors or the like using radio waves of the 920 MHz band is known. The 920 MHz band is a frequency band on which the ban has been removed from July, 2011 by the Ministry of International Affairs and Communications of Japan and anyone can use this frequency band without any permission or license. However, the maximum continuous transmission duration is limited to four seconds by the regulation (Association of Radio Industries and Businesses (ARIB) STD T-108). When the continuous transmission duration is shortened further to 0.2 seconds, for example, it is possible to allocate a larger number of channels and to transmit and receive signals with less interference.